In a consumer product, such as a VCR, a microcomputer system may drive a fluorescent display and control multiple functions such as on-screen programming, tuning and channel numbering, audio and video switching, servo mechanism control and motor control logic. When the AC supply is connected or the supply is resumed following a discontinuity, it is essential that the microcomputer be powered up or reset in a prescribed manner in order to avoid possible physical damage to the mechanism or recording medium, and user inconvenience and annoyance. In a consumer environment a VCR may be subject to AC supply interruption resulting from many causes, such as, plugging or unplugging the AC supply with the unit switched on, contact bounce on the AC power switch, AC supply drop outs caused by heavy load switching, lightning etc. Short term AC disconnects should not result in the VCR user settings being lost or the user having to manually reset the control system to restore operability. To satisfy these requirements it is often necessary to monitor for supply interruption and produce appropriate control signals such as stop, halt, watch, etc., and at the reappearance of the supply to generate a reset signal appropriate for the microcomputer system.
One method employed to detect AC supply interruption, and imminent power supply failure, is by monitoring one of the DC power supply output voltages. A drop in the monitored supply voltage signals to the microcomputer system that a power shut down is imminent. The microcomputer assumes an appropriate mode, i.e. stop, halt, watch, standby etc. wherein certain parameters values are stored in non-volatile memory. Usually the microcomputer power is sustained by a backup system such as a battery or a large value capacitor for a period sufficient to perform an orderly shut down. In a consumer product, such as a VCR, power supply loading may vary significantly with operational mode. Hence monitoring a DC power supply voltage to detect an external AC supply interruption, may prove insensitive if the detection level is set to cope with the normal supply load variations.
Monitoring an AC supply after rectification, filtering, and possibly regulation, will only detect an AC supply failure after an undesirably long period of time has elapsed. In fact the cessation of the AC input supply will be indicated by the drop, or progressive disappearance of the monitored DC supply. The time period between the AC supply failure and the DC supply disappearance may be maximized by the selective use of battery or capacitor back up of specific system elements. However, both batteries and large value capacitors may represent both cost and size/volume penalties.
FIG. 1 shows a prior art reset circuit for a key display board (KDB) microcomputer used in a consumer VCR. The purpose of this circuit is to monitor a power supply voltage and at power up, or after a power interruption, generate a microcomputer RESET pulse.
When the AC supply is connected or the supply is resumed, the power supply produces +5 volts which feeds the circuit. Transistor TR1 is off since the voltage on C1 is less than the voltage required to turn on the base emitter junction of the transistor. With TR1 off, the +5 volts supply voltage, will charge C2 via R5, causing the RESET line to be at the supply voltage. A microcomputer may typically be reset by a potential equal to supply voltage applied to the RESET input and it remains in the reset condition until the RESET line switches to zero volts.
To switch the RESET line to zero volts, capacitor C1 starts to charge to a voltage level Vc, determined by voltage divider R1 and R2. When the voltage on capacitor C1 exceeds the base emitter turn on potential of TR1, the transistor switches on, drawing current from the supply and C2. The discharge of C2 forces the RESET line to a low, near zero volt potential. With zero volts on the RESET line the microcomputer starts to execute the internal instruction set.
When the power is switched off or interrupted, and the +5 volt supply disappears, diode D1 provides a discharge path for C1. However, if the interruption is of short duration between e.g. 32 millisec. to 273 millisec., capacitor C1 will not discharge below the turn on potential of TR1 base emitter junction. Thus when the supply returns immediately, TR1 turns on, since the base emitter junction has remained conducting due to the potential on C1. With TR1 on, the circuit fails to produce a RESET pulse upon resumption of the supply voltage. For power interruptions that occur without the +5 volts supply disappearing the circuit again fails to produce a RESET pulse upon resumption of the supply voltage.
To overcome the limitations of AC power failure detection and to generate an earlier indication of AC failure, a power failure control circuit according to the invention is provided.